The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a camera to be built in a portable device including a cellular phone and a portable information terminal, a digital still camera, a security camera, a vehicle onboard camera, and a network camera.
In these years, in place of cellular phones that are intended mainly for making phone calls, so-called “smartphones”, which are multi-functional cellular phones that can be used to execute various application software as well as making phone calls, have been more widely used. By executing application software on a smartphone, the smartphone can be used, for example, as a digital still camera, a car navigation system, or the like. In order to execute the functions, most models of the smartphones have cameras, similar to the cellular phones.
Generally speaking, product groups of the smartphones are often categorized according to specifications thereof for beginners to advanced users. Among them, an imaging lens to be mounted in a product designed for advanced users is required to have a high resolution lens configuration so as to be also applicable to a high pixel count imaging element.
As a method of attaining the high-resolution imaging lens, there is a method of increasing the number of lenses that compose the imaging lens. However, the increase of the number of lenses easily causes an increase in the size of the imaging lens. Therefore, the lens configuration having a large number of lenses is disadvantageous for mounting in a small-sized camera such as the above-described smartphones. For this reason, an imaging lens has been developed so as to restrain the number of lenses as small as possible. However, with rapid advancement in achieving higher pixel count of an imaging element in these days, an imaging lens has been developed so as to attain higher resolution rather than attaining a shorter total track length thereof. For example, although it has been common to mount a conventional camera unit containing an imaging lens and an imaging element inside of a smartphone, nowadays, there is an attempt to attach a camera unit, which is a separate unit from the smartphone, to a smartphone, so that it is possible to obtain an image that is equivalent to that obtained by a digital still camera.
In case of a lens configuration composed of six lenses, since the number of lenses that compose an imaging lens is large, it is somewhat disadvantageous for downsizing the imaging lens. However, since there is high flexibility in designing, it has potential of attaining satisfactory correction of aberrations and downsizing in a balanced manner. For example, as an imaging lens having a six-lens configuration as described above, an imaging lens described in Patent Reference has been known.    Patent Reference: Japanese Patent Application Publication No. 2013-195587
The imaging lens described in Patent Reference includes a first lens that is positive and has a convex surface directing to an object side, a second lens that is negative and has a concave surface directing to an image plane side, a third lens that is negative and has a concave surface directing to the object side, a fourth lens and a fifth lens that are positive and have convex surfaces to the image plane side, and a sixth lens that is negative and has a concave surface directing to the object side. According to the imaging lens of Patent Reference, by satisfying a conditional relation concerning a ratio between a focal length of the first lens and a focal length of the third lens and a ratio between a focal length of the second lens and a focal length of the whole lens system, it is possible to satisfactorily correct a distortion and a chromatic aberration.
The cellular phones and smartphones have higher functions and smaller sizes each year and imaging lenses are required to have even smaller sizes than before. In case of the imaging lens of Patent Reference, the distance from the object-side surface of the first lens to the image plane of an imaging element is long. Therefore, to satisfy the above-described demands, there is a limit by itself in satisfactorily correcting aberrations while downsizing the imaging lens.
Here, such a problem is not specific to the imaging lens to be mounted in the cellular phones and smartphones. Rather, it is a common problem even for an imaging lens to be mounted in a relatively small camera such as digital still cameras, portable information terminals, security cameras, vehicle onboard cameras, and network cameras.
In view of the above-described problems in conventional techniques, an object of the present invention is to provide an imaging lens that can attain downsizing thereof and satisfactory aberration correction.
Further objects and advantages of the present invention will be apparent from the following description of the invention.